As used herein, the term LNG shall refer to Liquefied Natural Gas, that is Natural gas that has been cooled down such that it condenses and becomes liquid.
As used herein, the term Natural Gas shall refer to a gaseous mixture of hydrocarbons where an essential part is methane.
As used herein, the term LPG shall refer to Liquid Petroleum Gas, that is a gaseous mixture of hydrocarbons comprising propane and butanes.
As used herein, the term “mixed refrigerant cycle” shall refer to a liquification process, known in the art, employing an optimized mixture of a plurality of refrigerants.
As used herein, the term “gas expansion process” or “gas expansion cycle” shall refer to a liquefaction process, known in the art, employing a gaseous cooling agent, wherein the gaseous cooling agent at a higher pressure is first fed to a heat exchanger system and cooled but such that the cooled cooling agent is a gas. Thereafter the cooled cooling agent is expanded in a gas expander to a lower pressure lower than said higher pressure to generate a cold gaseous cooling agent stream. The cold gaseous cooling agent is led back to the heat exchanger system where it cools down the cooling agent stream at the higher pressure and for heat exchange with the fluid that is to be cooled, such as a gas to be liquefied, and such that the said cold gaseous cooling agent becomes a heated cooling agent. The heated cooling agent is thereafter compressed for reuse.
As used herein, the term “warm composite curve”, as known in the art, shall refer to the heat flow versus temperature relationship for the sum of warm streams being cooled down in a heat exchanger or a system of heat exchangers.
As used herein, the term “cold composite curve”, as known in the art, shall refer to the heat flow versus temperature relationship for the sum of cold streams being heated in a heat exchanger or a system of heat exchangers.
A used herein, the term “warm end” of a liquefaction heat exchanger, as known in the art, shall refer to the area or range of the heat exchanger where the warmest streams involved in the heat exchange is entering or leaving the heat exchanger.
As used herein, the term “split gas expansion cycles” shall refer to a gas expansion cycle wherein the cooled refrigerant is split into a plurality of streams, the streams being utilized at different stages and at different temperatures in the cooling of the target fluid.
As used herein, the term “fractionation column” shall refer to an arrangement, known in the art, for distillation separation of a mixed hydrocarbon fluid, in particular a column that generates an overhead fraction and bottom fraction
It is known in the art to produce LNG from a feed gas comprising a mixture of hydrocarbons, wherein the feed gas first passes through a fractionation column and an overhead fraction subjected to the liquefaction process, for example the system disclosed in EP 1715267. Such systems are employed in large scale, so-called “base load” liquefaction systems. Such systems typically employ a mixed refrigerant cycle, due to the superior efficiency of the mixed refrigerant cycle compared to the gas expansion cycle. Because the mixed refrigerant mixture is optimized, the overhead fraction must be cooled by an external source prior to being fed into the liquefaction circuit. As it is the intention of such systems to achieve an LNG product with as high a relative content of methane as possible, these systems are further arranged such that the bottom fraction from the fractionation column comprises a relatively high content of hydrocarbons heavier than methane.
The simplest way to limit the content of heavier hydrocarbons in the liquid gas is to partially condense the gas and then separate the condensed liquid from the gas, which is further cooled to be liquefied. The separation is normally carried out as an integrated part of the cooling down process at typical temperatures of between 0° C. and −60° C. Separated condensate can be heated up again as a part of the cooling process to utilise the cooling potential.
In large land based LNG installations (so called “base load” installations) most of the propane and heavier hydrocarbons are normally removed and in many cases also a considerable part of ethane, before or as a part of, the liquefaction. This is done to meet the sale specifications and to be able to produce and sell the valuable ethane, LPG and condensate/naphtha. Elaborate processes are normally used with low temperature fractionation columns both as a part of the cooling down process and as separate units outside the cooling system.
Because of the complexity of large, “Base load” systems, the arrangements used therein are not suitable for many applications, for example offshore applications. In addition, it is undesirable to handle products other than the LNG, as hydrocarbons lighter than C5 can, on the whole, not be stored or transported safely without being cooled down or under pressure.
In such offshore applications it is known to utilize the gas expansion cycle for the liquefaction of natural gas. The gas expansion cycle is relatively simple, but is less efficient than the mixed refrigerant cycle. While the use of the “split gas expansion cycle” can improve efficiency there is nonetheless a need for greater efficiency, as even relatively small changes in efficiency can result in very large economic gains.